FIG. 5 illustrates a prior art shaft double seal having two contact type mechanical seals 1a, 1b disposed in opposite directions. That is, in both mechanical seals stationary seal rings 2a, 2b are fixed relative to seal casing 5 and rotary seal rings 3a, 3b are slidable in the axial direction on rotary shaft 6, the seal rings 3a, 3b being pressed against stationary seal rings 2a, 2b by the thrust of springs 4a, 4b. The seal device is designed to sealingly separate the sealed fluid region A from the atmospheric region B by a purge fluid region C which is filled with a purge fluid L such as water or oil at an appropriate pressure. The term pressure as used herein means gauge pressure on the basis of atmospheric pressure.
In FIG. 5, the pressure P' of the purge fluid acts as a back pressure on the slidable seal rings 3a, 3b. Therefore, the purge fluid pressure P' is set higher than the fluid pressure P of the sealed fluid region by a specific amount .DELTA. P, where P'=P+.DELTA.P. This maintains appropriate contact between the seal rings 2a, 3a at the side of the sealed fluid region A. If the purge fluid pressure P' is lower than the sealed fluid pressure, the seal rings 2a, 3a cannot maintain appropriate contact. Therefore, the pressing force exerted by the spring 4a must be increased more than necessary.
However, in the mechanical seal 1a which provides the seal between sealed fluid region A and purge fluid region C, the purge fluid pressure P' must be set higher than the sealed fluid pressure P, and, when the sealed fluid is a volatile fluid or a low boiling point fluid such an ammonia, friction heat may break the fluid lubricating film between seal rings 2a, 3a. Once the seal is broken, purge fluid L leaks into the sealed fluid region A.
On the other hand, the mechanical seal 1b which seals the purge fluid region C from the atmospheric region B is a contact type seal. Therefore, its sealing function may be destroyed in high pressure conditions where the differential pressure (P+.DELTA.P) between the regions B and C is extremely high. Thus, the device of FIG. 5 is, generally speaking, not reliable. Depending on the properties and pressure of the fluid in the sealed region, it may be preferable to use another form of sealing device. In addition the structure of the device is complicated and peripheral equipment is required for circulating the purge fluid L.